It is well-known to form a variety of articles fabricated from plastic pellets. In the past, the practice generally was to extrude the plastic directly from the powder form in which it is recovered after polymerization. Because of the convenience of shipping and handling, it is desirable to form the plastic into pellets prior to final extrusion.
With the increased demand for pellets, the processing conditions to which pellets are exposed has become more demanding. When melt processed, conventional plastic pellets have a tendency to generate particulate degradation products (i.e., carbonaceous material, gels or fish eyes) in the extrudate, particularly when the plastic is exposed to relatively long residence times in the melt processing equipment.
Processing aids are conventionally blended into the plastic prior to fabrication into a pellet. However, it has been found that a certain lag time period exists, after exposure to desirable processing temperatures, before the blended processing aids function effectively. It is during the lag time period, that the plastic is particularly susceptible to decomposition upon exposure to desirable processing temperatures.
In some instances, plastic pellets have had processing aids applied to their surfaces. Generally, such processing aids are applied by spraying droplets of processing aid on the pellet surface. While such techniques have had some success, these techniques do not consistently provide uniform pellet surface coating.
Although conventionally coated pellets may be satisfactorily extrudable for a period, such pellets have not been found to be easily extrudable for commercially required periods. Specifically, when extruded, the non-uniform coating on the pellet surface causes variations in extrusion rate, torque within the extruder and pressure within the extruder. Moreover, the non-fused coatings tend to flake off the pellet surface.
It is desirable to produce a plastic pellet, having its surface coated with lubricant, which is capable of being extruded without causing variations in torque of the extruder screw, melt pressure or rate of extrusion of the pellet. It is to these goals that the present invention is directed.